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Abstract:

In a device processing method, a laser beam is applied to a wafer along
division lines from the back side of the wafer, thereby forming a
division start point inside the wafer along the division lines at a depth
not reaching the finished thickness of each device. A protective member
is attached to the front side of the wafer before or after performing the
division start points are formed. An external force is applied through
the protective member to the wafer, thereby dividing the wafer along the
division lines to obtain the individual devices. The back side of the
wafer is ground to remove the modified layers, and a silicon nitride film
is formed on at least the side surface of each device. The silicon
nitride film has a gettering effect and is formed on the side surface of
each device, which surface is formed by a cleavage plane.

Claims:

1. A device processing method for processing a plurality of devices
obtained by dividing a wafer composed of a silicon substrate and said
plurality of devices formed on a front side of said silicon substrate so
as to be partitioned by a plurality of division lines, said device
processing method comprising: a division start point forming step of
applying a laser beam to said wafer along said division lines from a back
side of said wafer, thereby forming a plurality of modified layers as a
division start point inside said wafer along said division lines at a
depth not reaching a finished thickness of each device; a protective
member attaching step of attaching a protective member to the front side
of said wafer before or after performing said division start point
forming step; a dividing step of applying an external force through said
protective member to said wafer after performing said protective member
attaching step and said division start point forming step, thereby
dividing said wafer along said division lines to obtain said individual
devices; a back grinding step of grinding the back side of said wafer
after performing said dividing step, thereby removing said modified
layers; and a silicon nitride film forming step of forming a silicon
nitride film on at least a side surface of each device after performing
said back grinding step.

2. The device processing method according to claim 1, wherein said
silicon nitride film is formed both on the side surface of each device
and on a back side of each device in said silicon nitride film forming
step.

3. The device processing method according to claim 1, wherein the
thickness of said silicon nitride film to be formed in said silicon
nitride film forming step is set to 6 to 100 nm.

Description:

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a device processing method
including the steps of dividing a wafer into individual devices and
forming a silicon nitride film on at least the side surface of each
device, thereby producing a gettering effect in each device.

[0003] 2. Description of the Related Art

[0004] A plurality of devices such as ICs and LSIs are formed on the front
side of a silicon substrate so as to be partitioned by a plurality of
crossing division lines, thus providing a wafer. The back side of the
wafer is ground to reduce the thickness of the wafer to a predetermined
thickness. Thereafter, the wafer is divided into the individual devices
by using a dicing apparatus and the devices thus obtained are used for
various electronic equipment or the like. As a method for dividing the
wafer into the individual devices, there has been proposed and put to
practical use a technique of focusing a laser beam inside the wafer along
the division lines to form a plurality of modified layers inside the
wafer along the division lines and next applying an external force to the
wafer to thereby break the wafer along the division lines, thus dividing
the wafer into the individual devices (see Japanese Patent No. 3408805,
for example). By using this technique, the width of each division line
can be reduced, so that more devices can be formed from one wafer,
thereby improving the productivity.

[0005] If the modified layer is left on the side surface of each device,
the die strength of each device is reduced. To cope with this problem,
there has been proposed a technique of grinding the back side of the
wafer after dividing the wafer into the individual devices, thereby
removing the modified layers (see Japanese Patent No. 4398686, for
example).

SUMMARY OF THE INVENTION

[0006] However, it is recognized that each modified layer has a gettering
effect such that it attracts heavy metal atoms such as copper atoms to
suppress the phenomenon that the heavy metal atoms are moved toward the
front side of the wafer (where the devices are formed) to cause a
reduction in function of each device. Accordingly, when the back side of
the wafer is ground after dividing the wafer into the devices, thereby
removing the modified layers, the side surface of each device formed by
dividing the wafer becomes a flat cleavage plane without a strain
especially in the case that the wafer is formed from a silicon substrate.
As a result, the gettering effect of each modified layer is lost to cause
a reduction in function of each device.

[0007] It is therefore an object of the present invention to provide a
device processing method which can ensure a sufficient gettering effect
without reducing the die strength of each device even when the modified
layers formed inside the wafer by laser processing for dividing the wafer
are removed by grinding.

[0008] In accordance with an aspect of the present invention, there is
provided a device processing method for processing a plurality of devices
obtained by dividing a wafer composed of a silicon substrate and the
plurality of devices formed on the front side of the silicon substrate so
as to be partitioned by a plurality of division lines, the device
processing method including a division start point forming step of
applying a laser beam to the wafer along the division lines from the back
side of the wafer, thereby forming a plurality of modified layers as a
division start point inside the wafer along the division lines at a depth
not reaching the finished thickness of each device; a protective member
attaching step of attaching a protective member to the front side of the
wafer before or after performing the division start point forming step; a
dividing step of applying an external force through the protective member
to the wafer after performing the protective member attaching step and
the division start point forming step, thereby dividing the wafer along
the division lines to obtain the individual devices; a back grinding step
of grinding the back side of the wafer after performing the dividing
step, thereby removing the modified layers; and a silicon nitride film
forming step of forming a silicon nitride film on at least the side
surface of each device after performing the back grinding step.

[0009] Preferably, the silicon nitride film is formed both on the side
surface of each device and on the back side of each device in the silicon
nitride film forming step. Preferably, the thickness of the silicon
nitride film to be formed in the silicon nitride film forming step is set
to 6 to 100 nm.

[0010] According to the present invention, the silicon nitride film having
a gettering effect is formed on the side surface of each device, which
surface is formed by a cleavage plane from which the modified layer has
been removed by the back grinding step. Accordingly, it is possible to
suppress the phenomenon that heavy metal atoms such as copper atoms are
moved in each device to cause a reduction in function of each device.
Further, in the case that the silicon nitride film is formed both on the
side surface of each device and on the back side of each device, the back
side of each device may be polished to remove a strain, thereby improving
a die strength, and at the same time the gettering effect can also be
produced.

[0011] Further, it was confirmed from a test that when the thickness of
the silicon nitride film is 6 nm or more, the gettering effect can be
exhibited, whereas when the thickness of the silicon nitride film is
greater than 100 nm, the die strength is reduced. Accordingly, by setting
the thickness of the silicon nitride film to 6 to 100 nm, the gettering
effect can be produced without reducing the die strength of each device.

[0012] The above and other objects, features and advantages of the present
invention and the manner of realizing them will become more apparent, and
the invention itself will best be understood from a study of the
following description and appended claims with reference to the attached
drawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a perspective view of a wafer;

[0014]FIG. 2 is a perspective view showing a protective member attaching
step;

[0015]FIG. 3 is a perspective view showing a division start point forming
step;

[0016]FIG. 4 is a sectional view showing modified layers formed inside
the wafer by the division start point forming step;

[0017] FIG. 5 is a schematic sectional side view showing a dividing step;

[0018]FIG. 6 is a sectional view showing grooves formed in the wafer by
the dividing step;

[0019]FIG. 7 is a perspective view of the wafer processed by the dividing
step;

[0022] FIG. 10 is an enlarged sectional view showing a condition where the
protective tape is sandwiched by ring members in the groove width
increasing step;

[0023] FIG. 11 is a schematic diagram showing the configuration of a
sputtering apparatus for performing a silicon nitride film forming step;

[0024] FIG. 12 is an enlarged sectional view showing a silicon nitride
film formed on the side surface and back side of each device;

[0025] FIG. 13 is a table showing the result of a gettering effect test;

[0026] FIG. 14 is a plan view showing the configuration of the wafer
subjected to a die strength test;

[0027] FIG. 15 is a perspective view showing the die strength test;

[0028] FIG. 16 is a sectional view showing the die strength test; and

[0029] FIG. 17 is a graph showing the result of the die strength test.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Referring to FIG. 1, there is shown a wafer WF. The wafer WF is
composed of a silicon substrate and a plurality of devices D formed on
the front side of the silicon substrate, i.e., on the front side W1 of
the wafer WF. These devices D formed on the front side W1 of the wafer WF
are partitioned by a plurality of crossing division lines L. The wafer WF
is to be cut along the division lines L to obtain the individual devices
D respectively corresponding to chips. The wafer WF has a thickness of
500 μm, for example. As shown in FIG. 2, with the device processing
method of the present invention, a protective tape T is attached to the
front side W1 of the wafer WF. A ringlike frame F is attached to the
peripheral portion of the protective tape T. Accordingly, when the front
side W1 of the wafer WF is attached to the protective tape T supported to
the frame F, the wafer WF is supported through the protective tape T to
the frame F in the condition where the back side W2 of the wafer WF is
exposed. There will now be described a method of dividing the wafer WF
along the division lines L to obtain the individual devices D and next
forming a silicon nitride film on the side surface of each device D.

(1) Division Start Point Forming Step

[0031] First, imaging of the wafer WF by an infrared camera (not shown) is
performed from the back side W2 of the wafer WF to thereby detect the
division lines L formed on the front side W1 of the wafer WF. As shown in
FIG. 3, a laser beam 100a having a transmission wavelength to the wafer
WF is applied by a laser head 100 from the back side W2 of the wafer WF
along the division lines L detected above as horizontally feeding the
wafer WF, thereby focusing the laser beam 100a inside the wafer WF. This
laser processing is performed under the following conditions, for
example.

[0032] Light source: YAG laser

[0033] Wavelength: 1064 nm

[0034] Spot diameter: φ2 μm

[0035] Average power: 1.2 W

[0036] Repetition frequency: 80 kHz

[0037] Feed speed: 100 mm/sec

[0038] This laser processing is performed along all of the division lines
L to thereby form a plurality of modified layers 101 as a division start
point inside the wafer WF along all of the division lines L as shown in
FIG. 4. The depth of each modified layer 101 from the back side W2 of the
wafer WF is set to a depth not reaching the finished thickness of each
device D. More specifically, in the case that the thickness of the wafer
WF is T1 and the finished thickness of each device D is T2 with reference
to the front side W1 as shown in FIG. 4, each modified layer 101 is
formed in a depth range of (T1-T2) with reference to the back side W2 of
the wafer WF. For example, in the case that T1 is 500 μm and T2 is 100
μm, each modified layer 101 is formed in a depth range of 400 μm
from the back side W2 of the wafer WF.

(2) Dividing Step

[0039] After performing the division start point forming step, the wafer
WF is divided along the division lines L to obtain the individual devices
D by using a dividing apparatus 7 shown in FIG. 5 to apply an external
force to the wafer WF. The dividing apparatus 7 includes a cylindrical
wafer supporting portion 70 for supporting the lower side of the wafer WF
(the protective tape T attached to the front side W1 of the wafer WF) and
a frame fixing portion 71 for fixing the frame F supporting the wafer WF
through the protective tape T. The frame fixing portion 71 includes a
cylindrical support 710 for supporting the lower side of the frame F and
a plurality of clamps 711 for pressing the upper side of the frame F. The
wafer supporting portion 70 and the frame fixing portion 71 are
relatively movable in the vertical direction.

[0040] As shown in FIG. 5, the wafer supporting portion 70 and the support
710 of the frame fixing portion 71 are initially set at the same level.
In this condition, the wafer WF is placed on the upper end of the wafer
supporting portion 70, and the frame F is fixed to the frame fixing
portion 71. More specifically, the protective tape T attached to the
front side W1 of the wafer WF is supported to the upper end of the wafer
supporting portion 70, and the back side W2 of the wafer WF is exposed,
or oriented upward.

[0041] When the wafer supporting portion 70 is relatively raised from the
frame fixing portion 71 as shown by a phantom line in FIG. 5, the
protective tape T is expanded. As a result, a tensile force is
horizontally applied to the wafer WF to thereby break the wafer WF from
the modified layers 101 as a division start point, thereby forming a
plurality of grooves 102 as shown in FIG. 6. As shown in FIG. 7, these
grooves 102 are formed along all of the division lines L to obtain the
individual devices D. Even after dividing the wafer WF, the individual
devices D remain attached to the protective tape T to maintain the form
of the wafer WF as a whole. While the protective tape T is attached to
the front side W1 of the wafer WF before performing the division start
point forming step in this preferred embodiment, the protective tape T
may be attached to the front side W1 of the wafer WF after performing the
division start point forming step and before performing the dividing
step.

(3) Back Grinding Step

[0042] After performing the dividing step, the back side W2 of the wafer
WF divided into the individual devices D is ground by using a grinding
apparatus 8 shown in FIG. 8. The grinding apparatus 8 includes a chuck
table 80 for holding the wafer WF and grinding means 81 for grinding the
wafer WF held on the chuck table 80. The grinding means 81 includes a
spindle 82, a mount 83 formed at the lower end of the spindle 82, a
grinding wheel 84 mounted on the lower surface of the mount 83, and a
plurality of annularly arranged abrasive members 85 fixed to the lower
surface of the grinding wheel 84.

[0043] In operation, the wafer WF supported through the protective tape T
to the frame F is held on the chuck table 80 in the condition where the
protective tape T comes into contact with the upper surface of the chuck
table 80 and the back side W2 of the wafer WF is therefore exposed. The
chuck table 80 thus holding the wafer WF is located below the grinding
means 81 so that the back side W2 of the wafer WF is opposed to the
abrasive members 85. In this condition, the chuck table 80 is rotated at
300 RPM, for example, in the direction shown by an arrow A1 in FIG. 8,
and the spindle 82 is also rotated at 6000 RPM, for example, in the
direction shown by an arrow A2 in FIG. 8. In this condition, the grinding
means 81 is lowered to bring the rotating abrasive members 85 into
contact with the back side W2 of the wafer WF, thereby grinding the back
side W2. During the grinding operation, the abrasive members 85 are in
contact with the back side W2 so as to always pass through the center of
rotation of the wafer WF. When the thickness of the wafer WF is reduced
to a predetermined thickness, i.e., the finished thickness T2 shown in
FIG. 4, by this grinding operation, the grinding means 81 is raised to
end the grinding operation.

[0044] As described above, each modified layer 101 shown in FIG. 4 is
formed at a depth not reaching the finished thickness of each device D.
Accordingly, when the thickness of the wafer WF is reduced to the
finished thickness T2 of each device D by the grinding operation, each
modified layer 101 is removed by the grinding operation. Accordingly, the
side surface of each device D is formed by only a flat cleavage plane. At
this time, the back side of each device D is referred to as a ground back
side D2'.

(4) Groove Width Increasing Step

[0045] This step is an optical step to be performed in the case that the
width of each groove 102 between any adjacent ones of the devices D is
not sufficient in forming a silicon nitride film on the side surface of
each device D in the following silicon nitride film forming step. As
shown in FIG. 9, a set of ring members 86 and 87 is prepared. The set of
ring members 86 and 87 has an inner diameter larger than the diameter of
the wafer WF and an outer diameter smaller than the inner diameter of the
frame F. As apparent from FIG. 9, the inner diameter of the outer ring
member 86 is slightly larger than the outer diameter of the inner ring
member 87. As shown in FIG. 9, the outer ring member 86 is pressed
against the protective tape T from the upper side thereof and the inner
ring member 87 is pressed against the protective tape T from the lower
side thereof in the condition that the ring members 86 and 87 are
disposed between the wafer WF and the frame F. As a result, the
protective tape T is sandwiched between the inner circumferential surface
of the outer ring member 86 and the outer circumferential surface of the
inner ring member 87 as shown in FIG. 10. Accordingly, the protective
tape T is expanded in the direction shown by an arrow A3 in FIG. 10 to
thereby increase the width of each groove 102 shown in FIG. 9.

(5) Silicon Nitride Film Forming Step

[0046] After performing the back grinding step or the groove width
increasing step, a silicon nitride film forming step is performed to form
a silicon nitride film on at least the side surface of each device D. The
silicon nitride film may be formed by sputtering, for example. A
sputtering apparatus 9 shown in FIG. 11 may be used for the formation of
the silicon nitride film by sputtering. The sputtering apparatus 9
includes a chamber 90 having a gas inlet 91 and a gas outlet 92. An anode
93 and a cathode 94 are accommodated in the chamber 90 so as to be
opposed to each other.

[0047] Before holding the wafer WF to the anode 93, the protective tape T
is cut at a position T3 shown in FIG. 10 to separate the frame F from the
wafer WF. As shown in FIG. 11, the wafer WF is held through the
protective tape T to the anode 93 in the condition where the ring members
86 and 87 and the peripheral portion of the protective tape T sandwiched
therebetween are fitted in an annular groove 93a formed on the lower
surface of the anode 93. In this condition, the central portion of the
protective tape T is in contact with the lower surface of the anode 93,
and the ground back side D2' of each device D is exposed. On the other
hand, a target 95 of SiNx as the material of the silicon nitride film is
held to the cathode 94. A magnetron cathode having a diameter of φ 4
inches, for example, may be used as the cathode 94. In operating the
sputtering apparatus 9, the chamber 90 is evacuated by removing inside
gases from the gas outlet 92, and Ar gas and N2 gas are next
introduced from the gas inlet 91 into the chamber 90. For example, the Ar
gas is introduced at a rate of 10 ml/min and the N2 gas is
introduced at a rate of 50 ml/min. The gas pressure in the chamber 90 is
set to 0.3 Pa, for example.

[0048] An RF voltage of 700 W, for example, is applied between the anode
93 and the cathode 94 to thereby generate a glow discharge. As a result,
argon ions Ar.sup.+ in the plasma collide with the target 95 on the
cathode 94 to eject target atoms 96 from the surface of the target 95.
The ejected target atoms 96 are attracted toward the anode 93, so that
the target atoms 96 enter each groove 102 between any adjacent ones of
the devices D to form a silicon nitride film 103 on the side surface D3
of each device D as shown in FIG. 12. At the same time, the target atoms
96 are also deposited on the ground back side D2' of each device D to
form a silicon nitride film 104 on the ground back side D2' of each
device D as shown in FIG. 12. In the present invention, it is essential
to form a silicon nitride film on at least the side surface D3 of each
device D. Accordingly, in the case that a silicon nitride film is not
formed on the ground back side D2' of each device D, a mask member may be
preliminarily attached to the whole surface of the ground back side D2'.
Since the side surface D3 of each device D is formed by a flat cleavage
plane, there is no or insufficient gettering effect on the side surface
D3. However, by forming the silicon nitride film 103 on the side surface
D3 of each device D, a gettering effect can be produced.

[0049] While the modified layers 101 are formed inside the wafer WF in the
division start point forming step in this preferred embodiment mentioned
above, the division start point forming step may be modified so that a
laser beam having an absorption wavelength to the wafer WF is applied to
the back side W2 of the wafer WF along the division lines L to perform
ablation, thereby forming a plurality of division grooves exposed to the
back side W2 along the division lines L. A modified layer is formed on
the side surface of each division groove. This ablation may be performed
under the following conditions, for example.

[0050] Light source: YAG laser

[0051] Wavelength: 355 nm (third harmonic generation of YAG laser)

[0052] Spot diameter: φ 5 μm

[0053] Average power: 5.0 W

[0054] Repetition frequency: 50 kHz

[0055] Feed speed: 100 mm/sec

[0056] As in the case of forming the modified layers 101 inside the wafer
WF, the depth of each division groove is set to a depth not reaching the
finished thickness T2 of each device D. Also in the case of forming the
division grooves on the back side W2 of the wafer WF by ablation, the
wafer WF can be divided into the individual devices D by applying a
horizontal tensile force to the wafer WF in the dividing step.

[0057] Also in the back grinding step, the back side W2 of the wafer WF is
ground to attain the finished thickness T2 of each device D. Since the
division grooves are formed at a depth not reaching the finished
thickness T2 of each device D, the division grooves are removed by this
grinding operation. Accordingly, the side surface of each device D is
formed by only a flat cleavage plane. Also in the following silicon
nitride film forming step, a silicon nitride film is similarly formed on
at least the side surface of each device D. As a modification, a back
polishing step of polishing the ground back side D2' of each device D to
remove a grinding strain may be additionally performed between the back
grinding step and the groove width increasing step or the silicon nitride
film forming step.

Example 1

[0058] A test was conducted to obtain the thickness of the silicon nitride
film for suitably ensuring the gettering effect of each device D. More
specifically, the back grinding step, the back polishing step to remove a
grinding strain, and the silicon nitride film forming step mentioned
above were performed to obtain various samples of each device D in which
silicon nitride films having different thicknesses were formed on the
side surfaces and back sides of the samples. These samples were used to
conduct a gettering effect test, thereby examining the relation between
the thickness of the silicon nitride film and the gettering effect of
each device. Further, it was found that the formation of a silicon
nitride film on each device causes a reduction in die strength. In this
respect, a die strength test was also conducted. In conducting the above
tests, the conditions of the wafer WF were set as follows:

[0059] Wafer: silicon wafer

[0060] Wafer diameter: 8 inches

[0061] Wafer thickness (device thickness): 500 μm (after polishing the
back side of the wafer)

[0062] Device size: 20 mm×20 mm

[0063] Number of devices per wafer: 61 (see FIG. 14)

(1) Gettering Effect Test

(A) Silicon Nitride Film Forming Step

[0064] A plurality of wafers subjected to the dividing step, the back
grinding step, and the back polishing step were prepared and the silicon
nitride film forming step was performed to form silicon nitride films
having different thicknesses of 1, 3, 5, 6, 7, 10, 50, 100, and 200 nm on
the side surfaces and back sides of the devices of these wafers. Further,
a wafer (device) without a silicon nitride film was also prepared, in
which no silicon nitride film was formed on the side surface and back
side of each device subjected to the back grinding step and the back
polishing step. All of these devices were subjected to the following
steps (B) to (D).

(B) Forced Contamination Step

[0065] A Cu standard solution (copper sulfate) was applied in an amount of
1.0×1013 atoms/cm2 to the back sides of all the wafers
each having a diameter of 8 inches, in which the silicon nitride films
having the above-mentioned different thicknesses were formed on the side
surfaces and back sides of the devices. Thus, forced contamination of the
devices with copper was made.

(C) Heating Step

[0066] After drying the coating of the Cu standard solution on the side
surfaces and back sides of all the devices, they were heated at
350° C. for three hours to obtain a condition where the copper
atoms in each device were easily diffused.

(D) Measuring Step

[0067] After cooling all the devices, the amount of copper atoms on the
other side (front side) of each device opposite to the back side coated
with the Cu standard solution was measured by using TXRF (total
reflection X-ray fluorescence analyzing apparatus manufactured by Technos
Co., Ltd.). More specifically, the front side of each wafer was
partitioned into a plurality of regions each having a size of 15
mm×15 mm, and the amount of copper atoms in each region was
measured to obtain the mean value and the maximum value of the amounts of
copper atoms in all of the regions. Further, also before performing the
forced contamination step, the amount of copper atoms on the front side
of each device was measured by a similar method.

[0068] In the case that copper atoms were detected on the front side of
each device in this step, it can be determined that copper atoms were
diffused in each device and that the gettering effect was zero or
insufficient. Conversely, in the case that copper atoms were not detected
on the front side of each device, it can be determined that copper atoms
were trapped on the silicon nitride film and that the gettering effect
was sufficient. The test result is shown in FIG. 13. The threshold
(detection limit) for determination whether or not copper atoms were
detected was set to 0.5×1010 atoms/cm2.

[0069] As apparent from the test result shown in FIG. 13, in the case that
the forced contamination step was performed and that the thickness of the
silicon nitride film was 5 nm or less, copper atoms were detected on the
front side of each device both at the mean value and at the maximum
value, so that it was confirmed that the gettering effect was zero or
insufficient. In contrast, in the case that the forced contamination step
was performed and that the thickness of the silicon nitride film was 6 nm
or more, copper atoms were not detected on the front side of each device
both at the mean value and at the maximum value, so that it was confirmed
that the gettering effect was sufficient (ND in FIG. 13 indicates that
copper atoms were not detected). Accordingly, it is considered that the
thickness of the silicon nitride film must be set to 6 nm or more to
ensure a sufficient gettering effect. Further, it is also apparent from
FIG. 13 that the larger the thickness of the silicon nitride film, the
better the gettering effect.

(2) Die Strength Test

[0070] As shown in FIG. 14, the wafer WF is composed of 61 chips numbered
by 1 to 61. After performing the silicon nitride film forming step, each
device of the wafer WF was subjected to the measurement of a die
strength. In the silicon nitride film forming step, a plurality of
silicon nitride films having different thicknesses of 0, 5, 10, 50, 100,
and 200 nm were formed. The measurement of a die strength was made by the
following specific method.

(E) Die Strength Measuring Step

[0071] The die strength of each device was measured by using a compression
tester (AGI-1kN9) manufactured by Shimadzu Corporation. A specific
measuring method for the die strength is as follows:

(E-1)

[0072] As shown in FIGS. 15 and 16, each of the chips 1 to 61 is placed on
a base 111 having a central circular hole 110 in the condition where the
silicon nitride film 104 formed on the back side of each chip is oriented
downward.

(E-2)

[0073] A pressure is applied to each of the chips 1 to 61 downward (in the
direction shown by an arrow A5 in FIG. 15) by using a pressure ball 112
having a spherical surface.

(E-3)

[0074] At the moment of fracture of each of the chips 1 to 61, a die
strength 5 is calculated by using Eq. (1) shown below

[0083] The definition and value of each variable in Eq. (2) are as
follows:

[0084] ε1: Young's modulus (silicon)=1.31×105 MPa

[0085] ε2: Young's modulus (pressure
ball)=2.01×104 MPa

[0086] r: Pressure ball radius=3.0 mm

[0087] v2: Poisson's ratio (pressure ball)=0.3

[0088] The die strength of each chip was measured to obtain a maximum
value, mean value, and minimum value. This measurement was made for the
different thicknesses of the silicon nitride films. As shown in FIG. 17,
the minimum value of the die strength (the minimum acceptable limit of
the die strength) is set to 1000 MPa, and the film thickness providing a
minimum value higher than 1000 MPa for the die strength is 0 to 100 nm.
In contrast, when the film thickness is 200 nm, the minimum value for the
die strength is lower than 1000 MPa.

(3) Optimum Film Thickness

[0089] As described above, the thickness of the silicon nitride film must
be set to 6 nm or more to ensure a sufficient gettering effect as
apparent from the result of the gettering effect test shown in FIG. 13.
Further, in order to ensure a sufficient die strength greater than an
acceptable limit, the thickness of the silicon nitride film must be set
to 0 to 100 nm. Accordingly, in order to ensure both a sufficient
gettering effect and a sufficient die strength, the film thickness of the
silicon nitride film to be formed on the side surface of each device must
be set to 6 to 100 nm.

[0090] The present invention is not limited to the details of the above
described preferred embodiments. The scope of the invention is defined by
the appended claims and all changes and modifications as fall within the
equivalence of the scope of the claims are therefore to be embraced by
the invention.